The present application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Patent Application No.2001-022565 filed on Jan. 31, 2001 (13th Year of Heisei), the entire content of which is incorporated herein by reference.
1. Field of the Invention
The present invention is generally directed to a thermoelectric module and method of producing the same, the thermoelectric module being in the form of, e.g., a semiconductor laser module for optical communication, a semiconductor amplifier module, an external modulator module or a receiving module.
2. Discussion of the Background Art
In general, the temperature of an installation environment of such a module or the external atmospheric temperature of a case in which such a module is installed reaches 60-70 degrees Centigrade due to the Joule heat of its electric components. Due to the fact that the laser diode which is widely used in a laser module as a light source in high-speed optical fiber communication changes its optical characteristics such as wavelength with changes in atmospheric temperature, the chip carrier which mounts the laser diode thereon is kept in thermal isolation from the case. In order to ensure such thermal isolation and to make the temperature of the laser diode constant, a thermoelectric module (i.e., temperature controlled thermoelectric module) has been used or employed.
This thermoelectric module is produced by soldering an electrode provided insulating material and a semiconductor chip, to connect the two. As Japanese Patent Laid-open Print No.Hei. 10 (1998)-62659 discloses, a soldered connection is provided at heat-absorption and heat-radiation sides of the insulating material, in general.
For example, for cooling, the soldered assembly of the thermoelectric module is soldered to an object to be cooled down or a structure to be heat-radiated such as a heat sink or box, using a soldering agent. This soldering agent is lower in melting point (solidus/liquidus), than a soldering agent which is used in the module assembly for the connection of semiconductor tip). This is done to prevent the thermoelectric module from being deformed or broken upon soldering connection. Selection of the soldering agent is determined in consideration of a safe rate of temperature overshoot upon heating, relative to the melting point of the tip connecting soldering agent.
When the thermoelectric module is required to be heat-resistant in-use, as in xe2x80x9cA-Z of Thermoelectric Conversionxe2x80x9d (1995, pp. 24-25, Realize Company), 95Sn5Sb soldering agent is used for the connection of the semiconductor tip in general, due to heat-resistance limitations of the semiconductor tip as well as the soldering agent, per se, respectively.
However, using the above-mentioned soldering agent which is lower in melting point (solidus/liquidus) than the soldering agent which is used in the module assembly for the connection of the semiconductor tip can lower the connection reliability at the connection of the structure to be heat-radiated when the module is required to be used in a severe thermal environment.
In addition, if 95Sn5Sb soldering agent is employed as the soldering agent which is used in the module assembly for the connection of semiconductor tip, making the other soldering agent as high as possible in heat resistance results in including Pb, which is environmentally unacceptable.
Thus, a need exists to provide a thermoelectric module which is free from the aforementioned drawbacks.
Accordingly in order to meet the above need to overcome the aforementioned drawbacks or problems, a first aspect of the present invention provides a thermoelectric module which includes a case, a heat-radiation side insulating substrate, a heat-absorption side insulating substrate, a first soldering layer formed of a first soldering agent at a position to connect the heat-radiation side insulating substrate and the case, a plurality of P-type and N-type semiconductor chips interposed between the heat-radiation side insulating substrate and the heat-absorption side insulating substrate, the plurality of P-type and N-type semiconductor chips being arranged alternately to be connected in a series, and a second soldering layer formed of a second soldering agent at a position to connect the heat-radiation side insulating substrate and one end of each of the plural P-type and N-type semiconductor chips, the second soldering layer connecting the heat-absorption side insulating substrate and the other end of each of the plural P-type and N-type semiconductor chips, the second soldering agent being identical with the first soldering agent in raw material.
In accordance with the first aspect of the present invention, even if 95Sn5Sb is employed as the second soldering agent for the connection of the semiconductors, forming the thermoelectric module can be made possible without lowering heat-resistance at the heat-radiation side. In addition, no other soldering agents make it possible to produce the thermoelectric module at a lower cost.
A second aspect of the present invention provides a method of producing a thermoelectric module which includes the steps of a first process for connecting a case and a heat-radiation side insulating substrate with a first soldering agent to form a first
soldering layer between the case and the heat-radiation side insulating substrate; and
a second process for connecting a heat-radiation side insulating substrate and a heat-absorption side insulating substrate to one end and the other end, respectively, of each of a plurality of P-type and N-type semiconductor chips with a second soldering agent which is
identical with the first soldering agent in raw material.
In accordance with the second aspect of the present invention, during soldering, the thermoelectric module is made free from thermal deformation or breakage. In addition, no other soldering agents make it possible to produce the thermoelectric module at a lower cost.
A third aspect of the present invention provides a thermoelectric module wherein the first soldering agent and the second are selected at least from 95Sn5 Sb, 91Sn9Zn, 96.5Sn3.5Ag, 97.5Sn2.5Ag, 100Sn, 65Sn25Ag10Sb, 99Sn1Sb, 90In10Ag, 97Sn3Sb, 95Sn5Ag, 93Sn7Sb, 80Au20Sn, 90Sn10Ag, and 97Sn3Cu.
In accordance with the third aspect of the present invention, forming the thermoelectric module can be done without lowering heat-resistance at the heat-radiation side. In addition, no Pb is contained in either soldering agent, which is environmentally desirable.
A fourth aspect of the present invention is to provide a method of producing a thermoelectric module wherein the first soldering agent and the second soldering are selected at least from 95Sn5Sb, 91Sn9Zn, 96.5Sn3.5Ag, 97.5Sn2.5Ag, 100Sn, 65Sn25Ag10Sb, 99Sn1Sb, 90Sn10Ag, 97Sn3Sb, 95Sn5Ag, 93Sn7Sb, 80Au20Sn, 90Sn10Ag and 97Sn3Cu.
In accordance with the fourth aspect of the present invention, during soldering, the thermoelectric module is made free from thermal deformation or breakage.